Fix: Adding the items to the listbox this way both gives me the Tag value and an Index value as well.
<ScrollViewer>
<ListBox Name='ListBox' BorderThickness='1.0' SelectedIndex='0'>
<ListBoxItem Content="Coffie" Tag="Coffee"></ListBoxItem>
<ListBoxItem Content="Tea" Tag="Tea"></ListBoxItem>
<ListBoxItem Content="Orange Juice" Tag="Orange Juice"></ListBoxItem>
<ListBoxItem Content="Milk" Tag="Milk"></ListBoxItem>
</ListBox>
</ScrollViewer>